skge: make support for old Genesis chips optional

The GENESIS boards are really old PCI-X boards that are rare.
Marvell has dropped support for this hardware and there is no reason
for most users to have to have this code.

Rather than riddling code with ifdef's make one macro and let
the compiler do the dead code elimination.  This saves about 15%
of the text size.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>